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    quoted from my text book:

    F = GMm/r^2

    "where r is the distance apart from their centres"

    g = F/m = - GM/r^2

    Using the equation above, calculate a value for the mass of the Earth (radius of Earth = 6.38 x 10^6m)

    M = gr^2/G = 9.81 x (6.38 x 10^6)^2 / 6.67 x 10^-11


    ... i thought r represented the distance from the centre of two masses (NOT the radius of an object?)
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    Hello Maths guy

    In this "g" is the acceleration due to gravity on the surface of earth.
    now if you place a point particle of unit mass (m) on the surface of the earth (M), the distance between the two masses = radius of Earth. I hope this clarifies.
 
 
 
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